The mission of Leach Botanical Garden is to maintain and enhance living collections of plants for the purpose of education, research, and conservation and to preserve the legacy of the Garden’s founders, John and Lilla Leach. The Garden is committed to providing positive experiences to the diverse community upon which it relies for support.

The property was originally part of a 320-acre donation land claim belonging to Jacob Johnson, one of the sawmill operators who was furnishing lumber for the very early homes in Portland. Johnson's land extended from Mt Scott down across the creek that was later named for him.

What is now the Manor House and Gift Shop was built in 1923. In 1931, John and Lilla Leach purchased part of what had been Johnson's property and named it Sleepy Hollow. John, a pharmacist, and Lilla, an accomplished botanist, devoted their land to their fascination with plants. Lilla discovered five plant species new to science. One of these species dates back 10 million years ago; it was given the Latinized form of its discoverer's name, Kalmiopsis leachiana. John Leach was also a talented craftsman and very proficient at metalwork (some of his work is on display at the Garden). He was active in the Oregon Arts & Crafts Society and served as president.